1. Field of the Invention
This invention relates to a tunneling magnetoresistive device mounted, for example, in magnetic reproduction apparatus such as hard disks or other magnetic detector devices. More particularly, the invention relates to a tunneling magnetoresistive device that is able to obtain a stable resistance variation rate and also to a method for manufacturing the same.
2. Description of the Related Art
It is known that a GMR (giant magnetoresistive) device exhibiting a giant magnetoresistive effect is used as a read-only head mounted such as in a hard disk device, and such a GMR device has high sensitivity.
Among GMR devices, there is known a spin valve film that is relatively simple in structure and is able to change in resistance by application of a weak external magnetic field. This spin valve film most simply has a four-layered structure.
FIG. 15 is a partial schematic view showing the structure of a spin valve film. FIG. 15 is a front view as seen from a side opposite to a recording medium.
Reference numerals 1 and 3 indicated in FIG. 15 are, respectively, ferromagnetic layers formed of an NiFe alloy, and a non-magnetic conductive layer 2 formed of Cu or the like is interposed between the ferromagnetic layers.
With this type of spin valve film, the ferromagnetic layer 1 is a layer called a free magnetic layer, and the ferromagnetic layer 3 is a fixed magnetic layer. Hereinafter, the ferromagnetic layer 1 is called free magnetic layer, and the ferromagnetic layer 3 is called fixed magnetic layer.
As shown in FIG. 15, an antiferromagnetic layer 4 formed of NiMn alloy is formed in contact with the fixed magnetic layer 3, and when annealed in a magnetic field, an exchange anisotropic magnetic field takes place between the fixed magnetic layer 3 and the anti-ferromagnetic layer 4, thereby causing the magnetization to be fixed along a height thereof (in a direction of Y in the figure).
On the other hand, the free magnetic layer 1 is influenced by a bias layer (not shown), so that its magnetization is uniformly arranged in a direction of track width (in a direction of X in the figure), thereby permitting the magnetization to be in the crossing relation between the fixed magnetic layer 3 and the free magnetic layer 1.
As shown in FIG. 15, electrode layers 5, 5 are provided at opposite sides along the direction of track width (in a direction of X in the figure) of the laminated film covering from the free magnetic layer 1 to the anti-ferromagnetic layer 4, respectively. It will be noted that the conductive layers 5, 5 are each formed of Cu (copper), W (tungsten), Cr (chromium) or the like.
With the spin valve film shown in FIG. 15, when the direction of magnetization of the free magnetic layer 1 varies by the influence of a leakage magnetic field from a recording medium such as of a hard disk, the electric resistance varies by the relation with respect to the fixed magnetization direction of the fixed magnetic layer 3, the leakage magnetic current from the recording medium can be detected according to the variation in voltage based on the change of electric resistance. The resistance variation rate (MR ratio) of the spin valve film ranges from approximately several to tens and several of percent.
The recent tendency toward high recording density results in the increasing areal density of a hard disk device. With a GMR device that is now in the main current, there arises a problem as to whether or not a further higher recording density (particularly, of 40 Gbits/inch2 or over) is enabled.
As a reproduction head substituted for the GMR device, attention has now been drawn to a tunneling magnetoresistive device. The resistance variation rate (i.e. a TMR ratio) of the tunneling resistive effect device arrives at several tens of percent, so that a very high reproduction output can be obtained in comparison with the GMR device.
FIG. 16 is a schematic view showing part of a structure of a conventional tunneling magnetoresistive device. FIG. 16 is a front view as seen from a side opposite to a recording medium.
Reference numerals 1 and 3 indicated in FIG. 16 are, respectively, a free magnetic layer and a fixed magnetic layer, like the spin valve film shown in FIG. 15, and an anti-ferromagnetic layer 4 is formed on and in contact with the fixed magnetic layer 3.
The differences in structure from the spin valve film reside in that an insulating barrier layer 6 made, for example, of Al2O3 (alumina) is formed between the free magnetic layer 1 and the fixed magnetic layer 3 and that the electrodes 5, 5 are provided on opposite sides in a direction vertical (in a direction of Z in the figure) to the film face of a multi-layered film covering from the free magnetic layer 1 to the anti-ferromagnetic layer 4.
With the tunneling magnetoresistive device, when a voltage is applied to the two ferromagnetic layers (i.e. the free magnetic layer 1 and the fixed magnetic layer 3), an electric current (tunnel current) flows through the insulating barrier layer 6 by the tunneling effect.
Like the spin valve film, the tunneling magnetoresistive device is so arranged that the magnetization of the fixed magnetic layer 3 is fixed in the direction of Y in the figure and the magnetization of the free magnetic layer 1 is arranged in the direction of X in the figure, and the direction of the magnetization varies by the influence of an external magnetic field.
In case where the magnetizations of the fixed magnetic layer 3 and the free magnetic layer 1 are anti-parallel to each other, the tunnel current is most unlikely to pass, with a maximum resistance value. Where the magnetizations of the fixed magnetic layer 3 and the free magnetic layer 1 are parallel to each other, the tunnel current is most likely to pass, with the resistance being minimized.
When the magnetization of the free magnetic layer 1 varies by the influence of an external magnetic field, a varied electric resistance is taken as a variation in voltage, thus permitting a leakage magnetic field from a recording medium to be detected.
The resistance variation rate (TMR ratio, or xcex94RTMR) in the tunneling magnetoresistive device is represented by 2PPPF/(1xe2x88x92PPPF) wherein PP represents a spin polarizability (i.e. the difference in number of electrons between the upspin and the downspin is normalized on the basis of the total number of electrons and this spin polarizability is hereinafter referred to simply as polarizability), and PF represents a polarizability of the free magnetic layer. As will be seen from the above equation, the resistance variation rate is determined by the polarizabilities of the ferromagnetic layers. As the polarizabilities increase, the resistance variation rate increases theoretically.
The tunneling magnetoresistive device per se, which is composed of the insulating barrier layer 6 interposed between the two ferromagnetic layers 1, 3, was known far back in the past. One of the reasons why the tunneling magnetoresistive effective device has never been put to practical use is that it is necessary to form the insulating barrier layer 6 that is thin enough to cause electrons to be tunneled, and the formation of such a thin uniform insulating barrier layer 6 is very difficult. For instance, thickness of the above-mentioned insulating barrier layer 6 is at several tens of angstroms.
In order to make the insulating barrier layer 6 thin, it has been conventional to form the insulating barrier film 6 according to the following procedure.
More particularly, after formation of an electrode layer 6 and a free magnetic layer 1 in this order as viewed from below, metallic Al is formed as a film on the free magnetic layer 1 such as by sputtering. Next, the metallic Al is oxidized according to a pure oxygen natural oxidation method or an oxygen plasma method to provide Al2O3, thereby forming an insulating barrier layer 6.
This procedure is advantageous, over the case where Al2O3 is directly formed as a film by sputtering using a target of Al2O3, in that there can be formed the insulating barrier layer 6 that has a reduced number of defects such as pinholes.
However, with the procedure of forming the insulating barrier layer 6 by oxidation of the metal layer as set out above, it is very difficult to control a pressure and a feed time of oxygen being fed. This leaves the deficiencies that not only the metal layer, but also the free magnetic layer 1 formed beneath the metal layer is oxidized and that the metal layer cannot be oxidized fully, so that part of the metal layer is left in the insulating barrier layer 6.
In this way, the complete oxidation of the metal layer alone is technically difficult, causing the resistance variation rate to be lowered owing to partial oxidation of the free magnetic layer and the residue of the metal layer. Thus, With a conventional type of laminated tunneling magnetoresistive device formed by laminating the free magnetic layer 1, the insulating barrier layer 6 and the fixed magnetic layer 3, it has been difficult to obtain satisfactory reproducibility.
It is accordingly an object of the invention to provide a tunneling magnetoresistive device, which can solve the problems of the prior art.
It is another object of the invention to provide a tunneling magnetoresistive device, which can be manufactured in a simple manner and in a highly reproducible way.
It is a further object of the invention to provide a method for manufacturing such a device as mentioned above.
The above objects can be achieved, according to the invention, by a tunneling magnetoresistive device, which comprises a barrier layer, at least two ferromagnetic layers provided on the insulating barrier layer and arranged side by side along the surface of the insulating barrier layer, and a pair of electrode layers, wherein at least the two ferromagnetic layers are kept away from each other at a given space sufficient to show a tunnel effect so that an electric current passing from one of the electrode layers toward one of at least the two ferromagnetic layers is allowed to pass via the insulating barrier layer to the other ferromagnetic layer.
This tunneling magnetoresistive device makes use of the effect that incident electrons from a ferromagnetic layer is transferable to another ferromagnetic layer opposite to an insulating barrier, i.e. a quantum tunnel effect, in the case where the incident electrons have only an energy level insufficient to pass over the insulating barrier from a classical point of view.
In the classical theory, it has been accepted that when V greater than E wherein a potential of an insulating barrier is taken as V and an energy of incident electrons is taken as E, the incident electron cannot pass through or over the barrier. Nevertheless, when the insulating barrier satisfies some requirements such as of a small thickness, it has been experimentally confirmed that an incident electron can pass through the barrier with a potential of V.
The tunneling magnetoresistive device shown in FIG. 13 includes an insulating barrier interposed between ferromagnetic body L and ferromagnetic body R, and electrodes connected to the ferromagnetic layer L and the ferromagnetic layer R, respectively.
The case where the incident electron energy is lower than a barrier potential, under which the incident electrons pass through the barrier, is one where, as shown in FIG. 13, the wave function of incident electrons and the wave function of transmitted electrons are superposed within a range of film thickness of the insulating barrier. In this case, the incident electrons pass through the barrier to cause a tunnel current to flow.
According to the above theory, where the wave functions of the incident electrons and transmitted electrons do no superpose, any tunnel current does not flow within the barrier, thereby not showing any tunnel effect. Whether or not the superposition of the wave functions takes place relates to the film thickness of the insulating barrier, under which as the film thickness increases, the superposition of the wave functions is more unlikely to occur.
As stated above, the prior art tunneling magnetoresistive device of the laminated type includes a three-layered laminate of ferromagnetic layer/insulating barrier layer/ferromagnetic layer. In order to appropriately show the tunnel effect, it is necessary to form a thin insulating barrier. In practice, however, it has been technically difficult to form a very thin insulating barrier.
In the practice of the invention, at least two ferromagnetic layers are set on an insulating barrier layer and arranged side by side along the direction of the surface of the insulating barrier layer.
At least the two ferromagnetic layers should be kept away from each other at a given space therebetween wherein the space is so determined as to show a so-called tunnel effect wherein an electric current passing from an electrode layer toward one of at least the two ferromagnetic layer is transmitted
The above arrangement is advantageous in that it is not necessary to form a thin insulating barrier layer. More particularly, at least two ferromagnetic layers are set at the insulating barrier layer so that the ferromagnetic layers are arranged side by side along the direction of film surface of the insulating barrier layer to establish a given a space between the ferromagnetic layers, and whether or not a tunnel current flows within the insulating barrier layer depends on the dimension of the space. If the space is large, any tunnel current does not flow within the insulating barrier layer, and any change in resistance (i.e., the TMR effect) does not take place.
The space between the ferromagnetic layers should be formed so narrowly as to show the tunnel effect. For instance, it is considered that the space should preferably be formed approximately at a film thickness of an insulating barrier layer of a known laminated, tunneling magnetoresistive device having three-layer laminate of ferromagnetic layer/insulating barrier layer and ferromagnetic layer.
In this way, the thickness of the insulating barrier layer does not directly influence the occurrence of the tunnel effect in the practice of the invention. The tunnel effect is determined depending on the space between the ferromagnetic layers on the insulating barrier layer. The ferromagnetic layers can be formed after the formation of the insulating barrier layer, so that when the insulating barrier layer is formed by oxidation of a metal layer, it is unnecessary to take the influence on the oxidation of the ferromagnetic layers into account. Thus, it is possible to establish an electric resistance or the like necessary for the insulating barrier layer through complete oxidation of the insulating barrier layer.
As will be apparent from the above, the insulating barrier layer formed by complete oxidation of a metal layer can be formed without influencing the oxidation of the ferromagnetic layers in the practice of the invention. In addition, since the insulating barrier layer can be formed as thick, the formation of the insulating barrier layer that has a reduced number of defects such as pinholes or the like is enabled. Thus, there can be manufactured a device, which is higher in reproducibility and more stable than known laminated, tunneling magnetoresistive device.
Moreover, in the practice of the invention, it is preferred that at least one of at least the two ferromagnetic layers consists of a fixed magnetic layer wherein magnetization is fixed in a given direction, and at least one thereof consists of a free magnetic layer. In this connection, it is more preferred that the free magnetic layer has such magnetization uniformly arranged in a direction crossing to the direction of magnetization of the fixed magnetic layer.
Further, it is preferred that an antiferromagnetic layer is formed in contact with the fixed ferromagnetic layer so that the magnetization of the fixed magnetic layer is fixed in the given direction through the exchange anisotropic magnetic field occurring between the antiferromagnetic layer and the fixed magnetic layer.
In the case, at least the free magnetic layer may be formed as exposing to the surface opposite to a recording medium.
The free magnetic layer may have an antiferromagnetic layer formed in contact therewith at least at one end thereof, so that the magnetization of the free magnetic layer is uniformly arranged in a direction crossing to the magnetization of the fixed magnetic layer owing to the exchange anisotropic magnetic field occurring between the free magnetic layer and the antiferromagnetic layer. Alternatively, the free magnetic layer may have a bias layer at opposite sides or at one side thereof along the direction of track width, so that the magnetization of the free magnetic layer is uniformly arranged in a direction crossing to the direction of magnetization of the fixed magnetic layer by the influence of a bias magnetic field from the bias layer.
In the practice of the invention, it is preferred to provide three ferromagnetic layers that include one free magnetic layer and two fixed magnetic layers and are so arranged that the free magnetic layer and the fixed magnetic layers are, respectively, provided at given spaces to permit an electric current from an electrode layer flows via the insulating barrier layer to one fixed magnetic layer-free magnetic layer-the other fixed magnetic layer in this order. Moreover, a pair of electrode layers should preferably be provided directly or indirectly at the fixed magnetic layers, respectively.
The reason why the three ferromagnetic layers are provided resides in that the resistance value is reduced by the resonance tunnel effect. The fundamental principle of the resonance tunnel effect in the case where the three ferromagnetic layers are provided is illustrated with reference to FIG. 14. FIG. 14 schematically shows a case where any bias voltage is not applied between the electrode layers.
As shown in FIG. 14, a barrier exists between ferromagnetic body L and ferromagnetic body M and also between the ferromagnetic body M and ferromagnetic body R, respectively, and electrodes are connected to the ferromagnetic body L and the ferromagnetic body R, respectively.
As shown in FIG. 14, there exist electron 1 and electron 2 having different energies in ferromagnetic body L.
Electron 1 is unlikely to pass through an insulating barrier, with its transmittance being very low. On the other hand, although electron 2 is lower in energy than electron 1, it can completely pass through the insulating barrier with its transmittance being at 1.
The reason why electron 2 of lower energy is more likely to pass through the insulating barrier in this way is ascribed to the fact that the energy (represented as bound level in FIG. 14) which an electron is able to take is discrete in ferromagnetic body M.
If this bound level and the energy of the incident electron 2 are resonant (coincident) with each other, the transmittance of the incident electron 2 comes to 1, thus permitting the resistance value to be lowered.
In the case where the three ferromagnetic layers are provided while making use of the above-mentioned resonance tunnel effect, two insulating barrier layers each having a small thickness have to be formed for a structure wherein ferromagnetic body L/insulating barrier/ferromagnetic body M/insulating barrier/ferromagnetic body R are laminated in this order from below, like a prior art device. This makes it more difficult to manufacture in comparison with a tunneling magnetoresistive device where only one insulating barrier layer 6 is formed as shown in FIG. 16, resulting in the considerable lowering of yield.
In contrast thereto, according to the invention, an insulating barrier layer has been previously formed by oxidation of a metal layer, and only three ferromagnetic layers are provided on the insulating barrier layer on the same plane, thereby forming a tunneling magnetoresistive device making use of the resonance tunnel effect. This makes it easier to manufacture over a prior art case, thus enabling one to manufacture the tunneling magnetoresistive device in high yield.
Furthermore, in the practice of the invention, three ferromagnetic layers are provided including two free magnetic layers and one fixed magnetic layer wherein the free magnetic layers and the fixed magnetic layer should preferably be so set as to make a given space between any adjacent layers in such a way that an electric current from an electrode layer flows via the insulating barrier layer in the order of one free magnetic layer-fixed magnetic layer-the other free magnetic layer. In addition, it is preferred that a pair of electrodes is, respectively, connected to the respective free magnetic layers.
In the invention, the antiferromagnetic layers are each formed of an Xxe2x80x94Mn alloy wherein X represents one or more of Pt, Pd, Ir, Rh, Ru and Os. These antiferromagnetic layers are advantageous in that they have a large exchange anisotropic magnetic field and a high blocking temperature.
Moreover, it is also preferred that the ferromagnetic layer is formed of either a ferromagnetic metal or half metal.
As stated hereinabove, the TMR ratio is theoretically determined by the polarizability of ferromagnetic layer. Accordingly, the use of a ferromagnetic layer of high polarizability is preferred from the standpoint of improving the TMR ratio.
In the practice of the invention, the ferromagnetic layers are formed of either a ferromagnetic metal or half metal wherein some kind of half metal has a polarizability at or near 1, so that when if a half metal is used for the ferromagnetic layers, it is theoretically possible to maximize the TMR ratio.
The ferromagnetic layers should preferably be formed of one or more of an NiFe alloy, a CoFe alloy, a Co alloy, a CoNiFe alloy.
Further, in the practice of the invention, the ferromagnetic layers should preferably made of either a ferromagnetic metal or half metal formed of a perovskite-type oxide represented by R1-xAxMnO3 wherein R represents one or more elements selected from trivalent rare earth metal ions such as La3+, Pr3+, Nd3+ and like, A represents one or more elements selected from divalent alkaline earth metal ions such as Ca2+, Sr2+, Ba2+ and the like, and X is a compositional ratio.
In the above formula, it is preferred that R is La3+, A is Sr2+, and the compositional ratio of x is at 0.17 or over.
Alternatively, it is also preferred that the ferromagnetic layers are formed of a Heusler""s alloy of A2MnX or AMnX wherein A represents one or more elements selected from Cu, Au, Pd, Ni and Co, and X represents one or more elements selected from Al, In, Sn, Ga, Ge, Sb and Si.
In the practice of the invention, the insulating barrier layer should preferably be formed of an insulating material of an oxide of a metal and/or a semiconductor.
More particularly, the insulating material should preferably be made of an oxide of at least one element selected from Al, Mg, Nb, Ni, Gd, Ge, Si and Hf.
Where the insulating barrier layer is formed of a metal oxide and/or an oxide of a semiconductor, the metal and/or semiconductor is sputtered to form a film, followed by oxidation of the metal and/or semiconductor layer according to a pure oxygen natural oxidation method or an oxygen plasma method, thereby forming an insulating barrier layer made of an oxide of the metal and/or semiconductor.
In the invention, the insulating barrier layer should preferably be made of a paramagnetic insulator formed of a perovskite-type oxide, R1-xAxMNO3 wherein R represents one or more trivalent rare earth metal ions such as La3+, Pr3+, Nd3+ and like, A represents one or more elements selected from divalent alkaline earth metal ions such as Ca2+, Sr2+, Ba2+ and the like, and X is a compositional ratio.
In this case, it is more preferred that R is La3+, A is Sr2+, and the compositional ratio of x is 0.26 or below. Alternatively, it is more preferred that R is Pr3+, and A is Ca2+.
Moreover, it is preferred that the insulating barrier layer should have a granular structure wherein fine granules of a metal are dispersed in an insulator matrix.
When the insulating barrier layer has a granular structure wherein fine granules of a metal are dispersed in a perovskite-type oxide, R1-xAxMnO3, or an insulator matrix and the insulating barrier layer is formed of Al2O3, the tunnel effect can be appropriately shown even though the space is formed wider than a required space to be kept between the ferromagnetic layers. This enables one to alleviate the processing accuracy, thus leading to ease in manufacturing process.
The method for manufacturing a tunneling magnetoresistive device according to the invention is characterized by comprising the steps of:
(a) forming a metal layer on a substrate;
(b) forming an insulating barrier layer by oxidation of the metal layer;
(c) forming a ferromagnetic layer on the insulating barrier layer; and
(d) forming, in the ferromagnetic layer, a space sufficient to show a tunnel effect so that an electric current tunnels or flows within the insulating barrier layer.
According to the above manufacturing method, the metal layer is initially formed on the substrate and oxidized to form the insulating barrier layer. Thereafter, the ferromagnetic layer is formed on the insulating barrier layer.
Accordingly, any ferromagnetic layer is not formed when the metal layer is oxidized. Thus, the metal layer can be oxidized without taking the influence on the oxidation of the ferromagnetic layer into consideration, making the manufacturing process easy or simple.
According to the invention, since the insulating barrier layer can be formed as thick, so that defects such as pinholes or the like are unlikely to occur in the insulating barrier layer, thereby forming a tunneling magnetoresistive effective device having stable characteristics.
The method for manufacturing a tunneling magnetoresistive device according to another embodiment of the invention is characterized by comprising the steps of:
(e) forming an insulating barrier layer of a paramagnetic insulator made of a perovskite-type oxide represented by R1-xAxMnO3 wherein R represents one or more elements selected from trivalent rare earth metal ions such as La3+, Pr3+, Nd3+ and like, A represents one or more elements selected from divalent alkaline earth metal ions such as Ca2+, Sr2+, Ba2+ and the like, and X is a compositional ratio;
(f) forming a ferromagnetic layer made of either a ferromagnetic metal or half metal consisting of R1-xAxMnO3 wherein R represents one or more elements selected from trivalent rare earth metal ions such as La3+, Pr3+, Nd3+ and like, A represents one or more elements selected from divalent alkaline earth metal ions such as Ca2+, Sr2+, Ba2+ and the like, and X is a compositional ratio; and
(g) forming, in the ferromagnetic layer, a space sufficient to show a tunnel effect so that an electric current tunnels or flows within the insulating barrier layer.
In this embodiment, after the formation of the ferromagnetic layer on the substrate in the step (f), a space is formed in the ferromagnetic layer in the step (g). Finally, the insulating barrier layer in step (e) may be formed on the ferromagnetic layer.
In the practice of the invention, a perovskite-type oxide represented by R1-xAxMnO3 is used to form the insulating barrier, for which it is preferred that R represents La3+, A represents Sr2+ and X is 0.26 or below. It is also preferred that R is Pr3+ and A is Ca2+. In this way, the perovskite-type oxide of the formula, R1-xAxMnO3, can be formed as a paramagnetic insulator.
Moreover, with the perovskite-type oxide of the formula, R1-xAxMnO3, used to form the ferromagnetic layer in the step (f), it is preferred that R represents La3+, A represents Sr2+ and x is 0.17 or over. This permits the perovskite-type oxide of the formula, R1-xAxMnO3, to be formed as a ferromagnetic metal.
In the above manufacturing method, both insulating barrier layer and ferromagnetic layer can be formed of a perovskite-type oxide of the formula, R1-xAxMnO3. Accordingly, the insulating barrier layer and ferromagnetic layer can be formed by epitaxial growth, and thus, a thin, smooth insulating barrier layer can be formed. Proper control of the compositional ratio in the perovskite-type oxide, R1-xAxMnO3, makes it possible to form an insulating barrier layer that is lower in electric resistance than Al2O3. In addition, when ferromagnetic layers formed on the insulating barrier layer can be formed at a wide space therebetween, an appropriate tunnel effect can be expected. Thus, processing accuracy can be alleviated, and the manufacturing procedure can be made easy along with an improved yield.